Heart rate variability refers to the variability of the time interval between heartbeats and may be mathematically defined as the one-sigma standard deviation of the heart rate about the mean heart rate value. A heart rate variability test is a reflection of a person's current health status. By taking heart rate variability tests over time, an individual is able to gauge improvement or deterioration in their health status. Such improvements or deterioration of health may result from a number of sources including, e.g., changes in lifestyle such as smoking cessation, starting an exercise program, surgery recovery, stressor additions or removals, diet changes. Thus, in this context, the HRV test may be used as a medical motivator. The HRV test may also be used as an early indicator diagnostic tool. For example, the HRV test has been demonstrated to have prognostic associations with future coronary disease and events.
Human sleep is described as a succession of recurring stages, including, inter alia, an awake stage, non-REM stages and the REM stage. The awake stage in this context is actually the phase during which a person begins the process of falling asleep. Sleep quality changes with the transition from one sleep stage into another. Significantly for purposes of this invention, the transition from stage to stage is marked with observable, though subtle, changes in bodily function, including heart rate variability.
Analysis of 24-hour HRV typically shows a nocturnal increase in the standard deviation of heart beat intervals. The heart rate is further known to decrease relatively rapidly as a person transitions from the awake stage to the non-REM stages. As the individual eventually transitions from the non-REM sleep stages to REM sleep, the heart rate becomes more erratic and the variability increases. There are several stages of REM sleep, each marked by changes in heart rate variability. The first REM stage typically lasts about 10 minutes, with each recurring REM stage lengthening, with the final stage lasting about one hour. The inventive monitor is capable of detecting the heart rate variability within each sleep stage as well as the transition from one sleep stage to the next, i.e., the transition from awake to non-REM sleep, the transition from non-REM sleep to REM sleep, the completion of an REM sleep stage and subsequent transition to the next REM sleep stage, and the deep slow wave stages (also referred to as delta wave or deep sleep) within non-REM sleep.
In addition, utilization of heart rate, heart rate variability, sleep stage patterns and pattern identification may be used to determine if the user is at risk of suffering from a wide variety of maladies or conditions relating in general to cardiovascular diseases or conditions and sleep breathing disorders or conditions. It would be highly desirable to have a device and method to identify certain maladies, conditions or related events (1) before they occur, (2) during the occurrence of the malady, event or condition, and/or (3) after the malady, event and/or condition has occurred to allow the user and/or health care professional to examine the data, identify the particular malady, event and/or condition, and take appropriate action to correct the problem.
The present invention addresses these concerns.